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Presentation on theme: "OSPF 1."— Presentation transcript:

1 OSPF 1

2 Today's Talk Introduction Distance Vector Protocol Link State Protocol
OSPF operation Neighbor & Adjacency OSPF in broadcast networks 2

3 Today’s Talk ... Exercise-1 Scalability Exercise-2 Redistribution
simple OSPF network Scalability Exercise-2 multiple areas Redistribution Exercise-3 redistribution of routes 3 4

4 Introduction Open Shortest First Path protocol Preferred IGP
The myth : OSPF is hard to use Evolved from IS-IS protocol Link state protocol 4

5 Distance Vector Protocols
Listen to neighboring routers install routes in table, lowest distance wins Advertise all routes in table Very simple Very stupid 5

6 Distance Vector Protocols
G E B H A 2 B 2 C 2 D 1 E sup F 1 G 1 H 1 I1 I F C A 1 B 1 C 1 G sup H 1 6

7 Link State Protocols information about adjacencies sent to all routers
each router builds a topology database a "shortest path" algorithm is used to find best route converge as quickly as databases can be updated 7

8 Link State Protocols D A 1 2 G E B 3 H I F C router 1 A, B, C, G, H
D, E, F, G, I A G D 8

9 OSPF Operation Every OSPF router sends out 'hello' packets
Hello packets used to determine if neighbor is up Hello packets are small easy to process packets Hello packets are sent periodically (usually short interval) 9

10 The Hello Packet Router priority Hello interval Router dead interval
Network mask List of neighbors FDDI Dual Ring Hello Hello 10 39 32

11 OSPF Operation Once an adjacency is established, trade information with your neighbor Topology information is packaged in a "link state announcement" Announcements are sent ONCE, and only updated if there's a change (or every 45mins...) 11

12 OSPF Operation Change occurs Broadcast change Run SPF algorithm
Install output into forwarding table 12

13 Neighbor Bi-directional OSPF communication
Result of OSPF hello packets Need not exchange routing information 13

14 Adjacency Between OSPF neighbors Exchange routing information
Point-to-point or Broadcast media Point-to-point - neighbors are adjacent Broadcast media - not all neighbors are adjacent 14

15 Broadcast Media - problems
N neighbors - order of N square adjacency Not optimal Wasted bandwidth Does not scale 15

16 Broadcast Media Select a neighbor - Designated Router(DR)
All routers become adjacent to DR Exchange routing information with the DR DR updates all the neighbors Scales Backup Designated Router 16

17 Neighboring States 2-way Router sees itself in other Hello packets
DR selected from neighbors in state 2-way or greater 2-way DR BDR 17 42 35

18 Neighboring States Full Routers are fully adjacent
Databases synchronized Relationship to DR and BDR Full DR BDR 18 43 36

19 LSAs Propagate Along Adjacencies
DR BDR LSAs acknowledged along adjacencies 19 45 38

20 General cisco Concepts:
Ability to run multiple routing protocols in the same router Same prefix could be learnt via more than one routing protocol Adminstrative distance is used to discriminate between multiple source Connected, Static and other dynamic protocols 20

21 Exercise - 1: cisco configuration
console ‘?’ for help command completion ‘show’ commands ‘show configuration’ ‘configuration’ command 21

22 Exercise - 1: cisco configuration
‘configure terminal’ Different modes Example ‘interface ethernet 1’ ‘ip address x.x.x.x m.m.m.m’ ‘exit’ to exit a mode ‘show ip route’ ‘show ip route x.x.x.x’ 22

23 Exercise - 1 Simple OSPF network ‘show ip route’
‘show ip ospf neighbor’ ‘show ip ospf int’ ‘show ip protocol’ ‘show ip ospf database’ many debug commands ‘debug ip ospf ?’ 23

24 Exercise - 1 Simple OSPF network
‘network x.x.x.x m.m.m.m area <area-id> ‘m.m.m.m’ wildcard mask ‘0’ do not care bit ‘1’ check bit ‘ ’ mask for exact match ‘network area 0’ ‘network area 0’ 24

25 Exercise - 1 Specify the network running ospf
Simple OSPF network Need to create OSPF routing process router configuration command ‘router ospf <process number> Specify the network running ospf Should also specify the area 25

26 Exercise1 Verification:
show ip ospf show ip ospf neighbor show ip route show ip route ospf show ip ospf interface show ip ospf database 26

27 Scaling OSPF Each link transition causes a broadcast and SPF run
OSPF can group routers to appear as one single router OSPF areas 27

28 OSPF areas (before) 28

29 OSPF areas (after) 29

30 Scaling OSPF Rule of thumb: no more than 150 routers/area
Reality: no more than 500 routers/area Backbone "area" is an area Always ‘area 0’ Proper use of areas reduce bandwidth & CPU utilization 30

31 Scaling OSPF Route could be summarized are area boundary
Instability is limited within each area Divide and conquer 31

32 Exercise - 2: OSPF with area
Use ‘network ... area <area-id> command Each interface only in 1 area Multiple areas per router If more than on area, the router should be in area 0 ‘area 0’ used for inter-area traffic 32

33 Redistribution injecting route from other protocol
Often useful during transition Need to be careful about feedback Need to set metric 33

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